In order to maintain air-conditioned spaces at comfortable and healthy conditions, it is necessary to dry the supply air to about 57° F. wet bulb. It is also desirable to have the supply air dry bulb temperature at least about 5° F. warmer (62° F. or higher). This air feels more comfortable, and also avoids moisture-saturated conditions in the supply duct.
Conventionally the air drying is done with 44° F. chilled water, supplied from electric powered mechanical vapor compression chillers. Those chillers create an unacceptably high and costly peak summer electric demand in many areas. When the air reheat is done with external heat input, the chilling demand is increased by the amount of reheat—a very wasteful practice.
The drying and cooling of the air could alternatively be done with a heat-activated liquid desiccant cycle. Those cycles are proven and effective in drying air (leaving it warmer and dryer). However their performance degrades markedly as they are pushed to conditions where the air is also cooled, e.g. to the 62° F. DB/57° F. WB supply air condition cited above, coupled with a realistic heat rejection temperature, e.g. 83° F. cooling water. The required regeneration temperature goes up, cycle losses magnify, and COP goes down, thus requiring more input heat at higher temperature.
Liquid desiccant drying systems are well established. Commercial vendors include Munters, Drykor, Kathabar, and Niagara Blower (the latter having just acquired Kathabar). The desiccant drying process leaves the air dryer but hotter.
There have been many efforts to use liquid desiccants as coolers vs. only drying. This entails cooling the dried air at least back to room temperature, and preferably below room temperature. Gommed and Grossman (2008) report performance of a system with adiabatic drying and cooling the desiccant with cooling water. It provides very good dehumidification but very limited cooling—cooling COPs range from 0.23 to 0.74.
Liu et al (2006) report performance of a system with diabatic drying followed by evaporative cooling (also referred to as adiabatic humidification). They achieve a cooling COP of 0.61 at 80° C. regeneration temperature when using 15° C. cooling water for heat rejection. Lowenstein et al (2005) report on the transport properties of a low flow diabatic absorber that is directly evaporatively cooled. Jones (2008) reports performance of that low flow unit—a cooling COP of 0.52 at 78° C. regeneration temperature when using 23° C. cooling water.
Numerous researchers have studied and reported upon the combination of a heat pump with a desiccant cooling cycle such that the cold end of the heat pump chills the dried air, and the hot end supplies the regeneration heat. This can be done with either a mechanical compression chiller or an absorption chiller. One example of this type of hybrid system using a mechanical chiller is found in Peterson et al (U.S. Pat. No. 4,941,324).
Wilkinson (U.S. Pat. No. 5,070,703) reports study results on a hybrid of a closed cycle LiBr absorption chiller and an open cycle liquid LiBr desiccant system, wherein both condenser heat and absorber heat from the absorption chiller are supplied to the desiccant regeneration process. The desiccant section of the hybrid cycle incorporates diabatic dehumidification followed by chilling of the ventilation air.
Schinner and Radermacher (1999) also report study results for an integrated absorption chiller/desiccant hybrid cycle. In their case it is a single effect ammonia-water absorption chiller. They model a “triple effect” cycle, i.e. with both condenser and absorber heat supplied to the regeneration process. The ventilation air is adiabatically dried in a desiccant wheel, then cooled by heat exchange with outdoor air, and finally evaporatively cooled. They report calculated COPs above 1.0, but only at return air temperatures higher than desired (above 60° F. wet bulb). The calculated absorption cycle COP is 0.293, and the absorber temperature is 192° F., inferring a driving heat temperature above 300° F.
What is needed is an air conditioning cycle that can be powered by low temperature waste heat or solar heat, that has high COP, and that has low parasitic power demand to run fans and pumps.